The evolution of atmospheric particulate matter in an urban landscape since the Industrial Revolution

Atmospheric particulate matter (PM) causes 3.7 million annual deaths worldwide and potentially damages every organ in the body. The cancer-causing potential of fine particulates (PM2.5) highlights the inextricable link between air quality and human health. With over half of the world’s population living in cities, PM2.5 emissions are a major concern, however, our understanding of exposure to urban PM is restricted to relatively recent (post-1990) air quality monitoring programmes. To investigate how the composition and toxicity of PM has varied within an urban region, over timescales encompassing changing patterns of industrialisation and urbanisation, we reconstructed air pollution records spanning 200 years from the sediments of urban ponds in Merseyside (NW England), a heartland of urbanisation since the Industrial Revolution. These archives of urban environmental change across the region demonstrate a key shift in PM emissions from coarse carbonaceous ‘soot’ that peaked during the mid-twentieth century, to finer combustion-derived PM2.5 post-1980, mirroring changes in urban infrastructure. The evolution of urban pollution to a recent enhanced PM2.5 signal has important implications for understanding lifetime pollution exposures for urban populations over generational timescales.

Pollution particles Fine-grained magnetite (0.2 to 5 μm) as identified in household dust (PSD MD and SP size fractions). Iron-rich spheres from the high temperature combustion of fossil fuels. Soot from petrol and diesel engines contain mainly 0.1 -1 m ferrimagnetic minerals. Combustion signals from aircraft engines are a mix of ferrimagentic and antiferromagnetic particles. Coal combustion produces characteristic fine (< 2 m) magnetite and haematite spherules (IAS).

Multi domain (MD) (> 20 m) [>10 m]
High SIRM/ARM ratios and low  ARM Pollution particles S-RATIO -0.7 to -1.0 demonstrates dominance of soft MD grains, typically relatively coarse IAS (> 2 m) from coal combustion. Metalrich brake and tyre abrasion particles from road vehicles and aircrafts display coarse ferrimagentic MD signal. Fe-rich spheres 2-70 m derived from traffic emissions.
Unweathered bedrock Low SIRM/LF quotients (< 10 kAm -1 ) and low FD% (<1).    Table S12. Elemental chemistry, size and morphology properties for PM from TEOM filter. Elemental weight (Wt%): the concentration of an element in weight percentage; area: area of the particle; aspect ratio: longest divided by shortest particle diameter (symmetrical features such as cubes of spheres have an aspect ratio close to 1); breadth: shortest particle diameter; equivalent circle diameter (ECD): the diameter of a circle that has the same area as the particle, regardless of shape.
ECD is used as a primary function of size in this instance as the FC method used spheres to calibrate size. The behaviour of particles in air is more relative to ECD than length; length: longest particle diameter; perimeter: distance of the outer boarder of the particle; shape: indicates the shape of the feature (a circle has a value of 1, whereas elongated and irregular particles have larger values).
A high-throughput, automated SEM particle analysis was applied to PM from the TEOM filter that had been sorted into < 2.5 m by FC. A total of 3679 particles were analysed for morphological features, size and chemistry (via EDS) to compliment manual characterisation of PM using SEM-EDS. The majority of particles detected had an ECD (equivalent circle diameter) < 5 m highlighting the use of FC to concentrate particles within the PMfine size fraction from bulk PM10 samples. The breadth of particles ranged from 0.05 m to 43.21 m. Coarse PM (> 20 m) are likely to represent particles clumped together during transfer onto filter papers for SEM analysis.
From the automated SEM data coarse (ECD > 2.5 m) and fine (ECD < 2.5 m) particles were separated to assess the chemical composition and morphology of particles. Smaller particles were detected however were below the BSE image threshold. Future analysis could incorporate these particles by adjusting detection threshold values. Particles were then classified based on their dominating chemistry.  The catchment of the pond is defined by highly vegetated steep margins. With a pond-to-catchment ratio of 1:1.4, the primary inorganic contribution to the pond is highly likely to be atmospherically derived. The pond (water surface) is situated at 55 m AOD (Above Ordnance Datum) and local bedrock is comprised of Sherwood Sandstone Group with over lying silts and mudstones from the Merica Mudstone Group and superficial Devensian till deposits. The pond was formed from a marl pit dug pre-mid 19 th century out of underlying Bollin mudstone (aka lower Keuper marl). Bollin mudstone is comprised of interlaminated gypsiferous (CaSO4) and anhydritic (CaSO4) mudstone and dolomitic siltstone (CaMg (CO3)2) with minor amounts of albite (NaAlSi3O8) and potassium-feldspar (KAlSi3O8). The examination of maps spanning from the present day to tithe publications (1844) provides confidence in the longevity and minimal disturbance of DDP and its surrounding catchment. Consultation with the landowner and ecologists from the local borough council, at time of core extraction, revealed that there has been no recorded work (such as dredging) performed at the pond.  137 Cs activity has a well-resolved peak between 7-8.5 cm that almost certainly records the 1963 fallout maximum from the atmospheric testing of nuclear weapons. D: The CRS (constant rate of supply) dating model was used to calculate 210 Pb dates, which were corrected using the 1963 137 Cs depth as a reference. Dates below the 210 Pb dating horizon of 12.5 cm were extrapolated back to the start of the sediment record via polynomial trend lines of age versus depth plots. Extrapolated dates and dry bulk density data were used to calculate sediment accumulation rates outside of the CRS model.   Due to the small size of OG pond, multiple cores could not be successfully retrieved without causing sediment disturbance to the sediment basin. A: Total 210 Pb activity reaches equilibrium with the supported 210 Pb at ~23 cm. B: Maximum unsupported concentrations < 60 Bq kg -1 indicate the 210 Pb dating horizon is unlikely to be more than 3 half-lives (60-70 years). C: A relatively defined peak at ~12 cm is very likely to represent the 1963 fallout maximum from atmospheric testing of nuclear weapons. D: 210 Pb dates calculated using the CRS model places the 1963 in good agreement with the 137 Cs record. Slightly higher sedimentation rates are observed post-1990, with relatively uniform sedimentation rates observed from the md-20 th century to 1990. An episode of brief rapid sediment accumulation is observed in the 1940s. Sedimentation rates prior to this are uncertain. E: Intra-site core correlations were determined using magnetic susceptibility profiles from multiple extracted cores (GWP2 and GWP3).